1. Field of the Invention
This invention is related in general to the production of aluminum-coated polymeric structures used as barrier films for packaging applications. In particular, the invention pertains to the inline passivation of aluminum layers vapor-deposited on polymer webs in vacuum.
2. Description of the Related Art
Metallized films produced in high volume for applications such as decorative, food, agricultural, biomedical and electronic packaging are usually evaluated for their appearance, conductivity, adhesion to the polymer substrate, and for their gas and vapor barrier properties. It is well known that all of these properties, as well as resistance to corrosion, depend on the substrate and the metallization conditions. The web metallizing industry has adopted inline high-speed plasma treatment of the substrate to improve the quality of metal deposition, such as to produce better aluminum metal nucleation, adhesion and barrier properties, but little or no attention has been paid to corrosion resistance other than with regard to the effects of the conditions of the metallization process (such as pressure, deposition rate, and aluminum wire purity).
In the normal process of manufacture of aluminum/polymer structures, the aluminum is deposited in vacuum over a pre-treated polymer web that is spooled in the vacuum chamber between a feed roller and a take-up roller. The aluminum is never exposed to oxidizing elements until it is removed from the vacuum chamber, normally in a humid environment. Such metallized webs (such as PET, PP and PE) are exposed to air during post-production slitting and other converting processes, and this has been assumed adequate to passivate the aluminum layer for long-term corrosion resistance. However, when the aluminum/polymer structure is unwound from the take-up production spool for post-production processing, it has been found that the aluminum film is often strongly bonded to the backside of the polymeric film and tends to peel off and damage the barrier properties of the metallized film, phenomena that are usually referred to in the art as “blocking” and “peel off,” respectively.
We discovered that these problems are due to the early formation of hydrated aluminum oxide, Al2O3.(H2O)n, resulting from the reaction of the aluminum layer with air and moisture adsorbed and absorbed in the web during deposition and prior to rewinding of the coated web into the take-up roll. Some web materials (like PET) have a high moisture content (higher than olefinic films, for example) that further accelerates the oxidation process in the wound roll. Therefore, even if the aluminum/polymer structure is unwound in a low humidity environment to minimize the formation of hydrated aluminum oxide, a considerable amount will already have been formed immediately after metallization inside the film roll.
Furthermore, because aluminum is highly reactive in its metallic state, it may also react with organic functional groups on the surface of the polymer film to form organometallic compounds which, under the pressure present between the layers of the roll, cause the aluminum to adhere to the backside of the film. Thus, when the film is unwound from the roll, aluminum that is strongly bonded to the backside of the film will peel off.
This invention is directed at providing a solution to these problems by perfecting the manufacturing process of metallized structures. In particular, the invention describes an inline passivation process that has been found to essentially solve the peel-off and corrosion problems described above.